The present invention relates to a fluid drive oil recovery process which utilizes an injection of substantially liquid CO.sub.2, surfactant and water into a subterranean oil reservoir. More particularly, the invention relates to such a process in which the surfactant selected for use is particularly insensitive to high temperature and high salinity and is tailored to suit the chemical properties of the oil and water in the reservoir to be treated.
Numerous patents describe materials and techniques pertinent to an oil recovery process involving an injection of CO.sub.2, surfactant and water. The U.S. Pat. Nos. 2,226,119; 2,233,381 and 2,233,382 describe polyalkoxylated alcoholic or phenolic surfactants which are generally useful in many aqueous liquid fluid drive oil recovery processes. Patent 2,623,596 indicates that an increased oil recovery may be obtained by a fluid drive process which injects highly pressurized CO.sub.2. U.S. Pat. No. 3,065,790 indicates that, in a fluid drive process, the cost effectiveness of highly pressurized CO.sub.2 may be increased by injecting a slug of the CO.sub.2 ahead of a cheaper drive fluid. U.S. Pat. No. 3,330,346 indicates that almost any process for forming foam within a reservoir may be improved by using as the surfactant "OK Liquid", a polyalkoxylated alcohol sulfate of a mixture of alcohols plus a fatty acid amide builder. U.S. Pat. No. 3,342,256 indicates that, in a fluid drive process, the oil-displacing efficiency of a CO.sub.2 slug may be increased by including water and a foaming surfactant within that slug. U.S. Pat. No. 3,529,668 indicates that, in a fluid drive process, the efficiency of a slug of foamed CO.sub.2 may be increased by displacing it with specifically proportioned alternating slugs of gas and liquid. U.S. Pat. No. 4,088,190 indicates that, in a fluid drive process, the heat stability and durability of a CO.sub.2 foam may be increased by using an alkyl sulfoacetate surfactant. U.S. Pat. No. 4,113,011 indicates that in a CO.sub.2 foam drive, the problems of low salt tolerance which are typical of both the surface-active sulfates of polyalkoxylated alcohols containing 10 to 16 carbon atoms recommended by U.S. Pat. No. 3,330,346 and the alkyl sulfoacetate surfactants recommended by U.S. Pat. No. 4,088,190 may be avoided by using a surfactant sulfate of a polyalkoxy alcohol containing only 8 or 9 carbon atoms and injecting that surfactant ahead of the CO.sub.2.
An article by T. M. Doscher and M. L. El Arabi (Oil and Gas Journal, Apr. 12, 1982, pages 144-151) "Scale Model Experiments Show How CO.sub.2 Might Economically Recover Residual Oil" describes how field tests have shown that CO.sub.2 can mobilize and displace oil but so far the results "provide little support for the conclusion that carbon dioxide can economically recover residual oil". The authors state that "the efficiency of the process appears to depend primarily on the displacement of mobile water to expose the occluded oil to the carbon dioxide". They also mention how the problems due to gravity layover and reservoir heterogenity are magnified by the high mobility of CO.sub.2 and state that "practical methods for decreasing the mobility of carbon dioxide have not yet been developed" (page 150, last paragraph).
Thus, it is known to inject substantially liquefied CO.sub.2, surfactant and aqueous liquid into a subterranean reservoir to displace oil within the reservoir. It is known that where the salinity of the water available for use in such an injection and the temperature of the reservoir are suitably low, a selection can be made of a polyalkoxy alcohol nonionic surfactant which is particularly effective for decreasing the mobility of the CO.sub.2 in the reservoir. The '266 patent discloses that such a nonionic surfactant is one which is capable of forming each of three specified types of dispersions in contact with the reservoir oil and the water to be injected, at the reservoir temperature and a CO.sub.2 -liquefying pressure. It is further known that, in general, water-soluble polyalkoxy sulfonate surfactants have good chemical stability and good emulsifying capabilities, even at relatively high temperatures and high water salinities.
However, particularly with respect to oil reservoirs which are relatively hot and/or the water available for injecting is highly saline, it was not previously known that the above criteria or any criteria might identify a combination of water-soluble polyalkoxy sulfonate surfactant and aqueous liquid which, when injected along with substantially liquefied-CO.sub.2, would significantly reduce the mobility of the CO.sub.2 in a manner capable of increasing the preferential displacement of water in the reservoir and increasing the ratio of oil displaced-to-amount of CO.sub.2 injected.